1. Field of the Invention
The present invention relates to a storage battery voltage control apparatus for equalizing the voltage of each of a plurality of series-connected cells of a storage battery.
2. Description of the Related Art
A hybrid vehicle equipped with an internal combustion engine and an electric motor as driving power sources is known which comprises a storage battery including a plurality of series-connected storage battery cells as a power supply for driving the motor as disclosed in JP-A-7-163014.
In this storage battery, the performance variations (including those of capacitance, self-discharge current and internal resistance) causes an uneven voltage of each storage battery cell, sometimes resulting in the fact that a storage battery cell high in voltage reaches a maximum voltage (i.e., cannot be charged any higher) earlier than others at the time of charging, thus narrowing the working voltage range of the storage battery as a whole or reducing the durability thereof. In view of this, the above-mentioned conventional storage battery adopts such means as connecting a zener diode to a storage battery cell which reaches the maximum voltage early or connecting a resistor or a capacitor to a storage battery cell likely to reach a maximum voltage thereby to suppress the voltage increase thereof.
The apparatus described in JP-A-7-163014, however, is intended to equalize the voltage of each storage battery cell when the storage battery is fully charged and that apparatus cannot exhibit the desired effect thereof when used continuously at an intermediate voltage. Also, since the performance difference between the individual storage battery cells are not compensated for, the working voltage range may be narrowed by a voltage difference which may develop among the storage battery cells during a long-time operation.